A simpler solution requiring low tech equipment and minimal cost is as
follows:

Buy two wingnuts and carriage bolts about 3 inches long, two springs
somewhat stronger than those supplied for the rudder pedals. Grind off the
head flanges on the carriage bolts, drill a hole in the square shank for one
end of the spring.  manufacture an aluminum bracket to attach onto the pedal
end of the spring attach point.  the bracket hole for the bolts should be
about 1.5 -2 inches aft to allow adjustment room for the differential
tension. Install the springs to the pedal assembly, the bolt and use the
wing nuts for adjustment.

I have found that you only need to adjust the trim for cruise.  Some pilots
say that different gross weights affect trim.  I have found that to be
insignifcant.  Once adjusted just fly the bird. I am able to adjust the wing
nuts in flight. These birds require very little pedal to fly straight.  In
fact the other day I did a 60 degree banked turn without pedal pressure and
it flew nearly true with less than 1/4 ball movement.  Is this typical.  I
do not know.

I did not fininsh the plane before flying.  There were initial anomalies in
the rudder and stab surface which affected yaw.  The rudder was concave on
the left side and the stab was flat on the right side which caused a left
turn. remedial repairs to true up the surfaces provided proper trail of the
rudder, and straight flight not requiring a trim tab.  The differential
rudder pressure by adjusting the spring tension was minimal.

Build it straight and it will fly straight.

Good Luck

Bob Smiley